The wave theory of
light:
Thomas Young's "epoch-making contribution to the theory of
light"
and the first statement of his interference theory and
"double-slit" experiment

"Whatever opinion may be entertained of the theory of light and colours which I have lately had the honour of submitting to the Royal Society, it must at any rate be allowed that it has given birth to the discovery of a simple and general law, capable of explaining a number of phenomena of coloured light, which, without this law, would remain insulated and unintelligible.

The law is, that 'whenever two portions of the same light arrive at the eye by different routes, either exactly or very nearly in the same direction, the light becomes most intense when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions; and this length is different for light of different colors.'

I have already shown in detail, the sufficiency of this law for explaining all the phenomena described in the second and third books of Newton's Optics, as well as some others not mentioned by Newton." -Thomas
Young, p.387.

YOUNG,
Thomas. The Bakerian Lecture: "On the theory of light and colours,"
in Philosophical Transactions of the Royal Society, pts 1 & 2, pp
12-48; WITH: "An
account of some cases of the production of colours, not hitherto
described", in Philosophical Transactions, pp. 387-97. London: G.
and G. Nicol, 1802. Quarto, modern calf in period style. $6500.

First printing of two of the most
fundamental papers in modern physics: Thomas Young's evidence of the
wave theory of light and the first statement of his interference
principle and double-slit experiment, the most influential experiment in
quantum theory.

"The Bakerian Lecture ["On
the theory of light and colours"] delivered in November 1801 is an
epoch-making contribution to the theory of light in all its phases.
Hooke, Huygens and above all Newton had discussed the nature of light in
the seventeenth century. Huygens propounded the wave theory in 1690,
whereas Newton was predominantly in favour of a corpuscular theory...
Though criticized by Euler and some others, the corpuscular theory held
the field almost throughout the eighteenth century; but Young... based
himself firmly on the theory that 'radiant light consists of undulations
of the luminous ether': a theory that held the field until the
latter-day notions of Planck and J.J. Thomson." (Printing and
the Mind of Man, 259.)

Young opens "An account of some
cases of the production of colours" with the clear statement of
what would come to be known as Young's interference principle, giving
definitive evidence for the wave theory of light. He then describes an
experiment he performed that has since become canonical in the study of
modern physics:

"I therefore
made a rectangular hole in a card, and bent its ends so as to
support a hairparallel to the sides of the hole; then,
upon applying the eye near the hole, the hair, ofcourse,
appeared dilated by indistinct vision into a surface, of which
the breadth wasdetermined by the distance of the hair
and the magnitude of the hole, independently of thetemporary
aperture of the pupil. When the hair approached so near to the
direction of themargin of a candle that the inflected
light was sufficiently copious to produce a sensibleeffect,
the fringes began to appear; and it was easy to estimate theproportion of their breadth to the apparent breadth of the
hair across the image of whichthey extended. I found
that six of the brightest red fringes, nearly at equal distance,occupied the whole of that image. The breadth of the aperture
was 66/1000 [of an inch],and its distance from the hair
8/10 of an inch; the diameter of the hair was ... 1/600 [of aninch]. Hence, we have 11/1000 for the deviation of the first
red fringe at the distance of8/10; and as 8/10 / 11/1000
= 1/600 / 11/480000, or 1/43636 [of an inch] for thedifference
of the routes of the red light where it was most intense."

The experiment was later modified and
expanded by Young to allow light to pass through two pinholes onto a
screen and became known as the famous "double-slit"
experiment, often called the most beautiful experiment in physics. The
results of the experiment- namely that the light produced wave-like
interference patters that were, later shown, to be dependent on the light's
"knowledge" of which slits were open- were profound.
Richard Feynman was often quoted as saying that all the issues of
quantum mechanics can be understood by carefully thinking through the
implications of Young's experiment. Occasional light browning, paper
flaw to corner of one leaf of first Young paper (not affecting text). Rare.